Substrate  Pgp Efflux to Apical Chamber k_{2}^{a}  Pgp Dissociation to Apical Membrane k_{r}^{b}  Pgp Binding Constant K_{C} (Cytosolic Dissociation Constant K_{D, Aq})^{c}  Partition Coefficient K_{PC}^{d}  SteadyState Passive Permeability Coefficients^{e}  V_{max}/K_{m}  

Basolateral Transporter (Passive Transport into Cytosol)^{f}  Apical Transporter (Passive Transport into Cytosol)^{f}  
s ^{1}  M ^{1}  nm/s  s^{1} (%)  
AMP  150  2 × 10^{6}  1000 (5 μM)  200  P_{BA} = 420 ± 50  0 (0)  0 (0)  
P_{AB} = 400 ± 50  
QND  10  1 × 10^{5}  10,000 (0.1 μM)  700  P_{BA} = 500 ± 100  0 (0)  0 (0)  
P_{AB} = 500 ± 100  
LPM  1  1 × 10^{5}  20,000 (0.01 μM)  3000  P_{BA} = 350 ± 80  100 ± 20 (40)  0 (0)  
P_{AB} = 400 ± 100  
DGX  10  ND^{g}  ND^{g}  ND^{g}  P_{BA} = 30 ± 8  30 ± 10 (70)  2 ± 1 (5)  




 P_{AB} = 25 ± 5 


AMP, amprenavir; QND, quinidine; LPM, loperamide; DGX, digoxin; ND, not determined.
↵ a Estimate for the efflux rate constant k_{2}, from Pgp into the apical chamber, given by the ratio of the fitted k_{2}T_{0} for each drug and the center of the box value of T_{0} = 2 × 10^{4} M.
↵ b Estimate for the dissociation rate constant k_{r}, from Pgp back into the inner apical monolayer.
↵ c The binding constant between Pgp and the inner apical monolayer shown is the average of the best fits (> 160 for each drug concentration). Below each binding constant, we show in parentheses the appropriate dissociation constant for each drug relative to the aqueous phase, calculated as K_{D, Aq} = 1/(K_{C} * drug partition coefficient [phosphatidylserine (PS)/phosphatidylethanolamine (PE)/cholesterol (chol)]), the liposome mimic for the inner apical monolayer.
↵ d Equilibrium drug partition coefficient to 0.1 μ m PS/PE/chol (1:1:1) liposomes. This lipid composition is a rough mimic for the inner apical monolayer, as described in Tran et al. (2005).
↵ e Steadystate passive permeability coefficients measured across the confluent cell monolayer in the presence of the Pgp inhibitor GF120918. P_{BA} and P_{AB} are shown. The values are not always symmetric.
↵ f The fits for the other transporters. Both V_{max} and K_{m} were independently fitted, but only their ratio was constant. Transport is the fraction of transport into the cytosol, B>C or A>C, due to the transporter, relative to all +GF120918 passive transport.
↵ g The mass action equations only fit the product K_{C} * K_{PC}, which for digoxin is ∼5 × 10^{4} M^{1}. This value is similar to that for amprenavir. The partition coefficient for digoxin has not been measured, so we cannot separately estimate the binding constant, K_{C}, to Pgp or the dissociation rate constant k_{r} = k_{1}/K_{C}.